Impulse generator



y 13, 1947- s. DARLINGTONI 2,420,302

IMPULSE GENERATOR Filed Aug. 19, 1943 3 Sheets-Sheet 1 4121: u: 2/5 l L:1 I 11-1 /0 l2 4" 4n. ART. I

u/vs u: LINE FIG. 3 FIG. 2

TTTTT SYUMETRICAL NETWQRA l8 HAVING AN OPEN CIRCUIT IMPEDANCE 2,, ANDmuvsren consul/r a IDEAL 7RAN3FORMER F/G. 6 K 30 IR SYMMETR/CAL wsnmnxmum/a AN OPEN CIRCUIT IMPEDANCE 2 -.'-Cl-! A) AND TRANSFE CONS TANT lNlEN TOR S. DARLING TON A TTORNEY May 13, 1947; s. DARLINGTON IMPULSEGENERATOR.

Filed Aug. 19, 1943 3 Sheets-Sheet 3 FIG. /2

LINE w FIG. /3

LINE w FIG. /4

TTTTTT TT TTT SOURCE- INVENTOR 5. RL ING TON A TTORNE Y Patented May 13,1947 IMPULSE GENERATOR Sidney Darlingtcn, New York, N. Y., assignor toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application August 19, 1943, Serial No. 499,193

6 Claims.

This invention relates to impulse generating systems and moreparticularly to circuits for generating impulses of controlled wave formand duration. Its principal objects are to facilitate the voltagetransformation of an impulse while at the same time maintaining its waveform; to provide for voltage multiplication Without recourse to the useof electromagnetic transformers; and to increase the useful energy ofgenerated impulses.

In impulse transmission systems such, for example, as radio echosystems, in which high frequency impulses of very great intensity and ofshort duration are radiated at regular intervals, it has been founddesirable that the individual impulses should have a substantiallyrectangular wave form, that is, one characterized by uniform amplitudethroughout the whole of the impulse duration and by sharp growth anddecay at the beginning and at the end of the impulse, respec tively. Theradiated impulses are ordinarily produced by exciting an ultrahighfrequency vacuum tube oscillator with unidirectional voltage impulses ofthe desired shape and intensity, the radiated energy being derivedalmost wholly from that contained in the exciting pulse. I-Ieretoforethe generation of impulses of suitable shape and of sufilcient intensityhas presented a problem of considerable magnitude for the reasons thatdirect currents of the required voltage are not readily obtained andthat electromagnetic transformers capable of providing the necessaryvoltage transformation without impairing the impulse wave form arecostly and difficult to construct.

The present invention overcomes the abovementioned difficulties bymaking use of wave reflection effects to bring about the desired voltagestep-up. For this purpose, a generating impulse is caused to traverse achain of artificial lines having different characteristic impedanceswhich are graded in a particular manner as hereinafter described. Thewave reflection eiiects occur at the junctions of the several lines andI have found that, by suitably proportioning the line characteristics,it is possible to ellect any desired degree of voltage multiplicationwhile maintaining a rectangular wave form.

The nature of the invention will be more fully understood from thedetailed description which follows and by reference to the accompanyingdrawings of which:

Fig. 1 is a schematic representation of an impulse generating system inaccordance with the invention;

Figs. 2, 3 and 4 represent configurations of artificial lines andimpedance networks that may be used in the system of Fig. 1;

Figs. 5 and 6 illustrate a theorem upon which an explanation of theinvention is based;

Fig. 7 represents schematically a known type of impulse generator whichis a prototype of the systems of the invention;

Figs. 8 to 13, inclusive, illustrate the derivation of multiplyingsystems in accordance with the invention from the prototype circuit; and

Fig. 14 shows in more detail a voltage tripling system in accordancewith the invention.

Referring to Fig. 1, ill is a source of direct current, II a resistor,12 a switch, elements L1 to Ln, inclusive, are sections of artificiallines having negligible energy dissipation, i3 is a thermionic diode, I4is" a diode oscillator, for example, of the type shown in United StatesPatent 2,063,342, issued December 8, 1936, to A. L. Samuel, and i5 is adipole antenna coupled to the oscillator by a small coupling loop [6inserted in the oscillator field. The source It has its negativeterminal grounded. Diodes l3 and M are poled in opposite directions, theformer in conductive relation to the battery and the latter inopposition thereto. The cathode heating circuits for these vacuum tubesare omitted for the sake of simplicity. Lines L1 to lit-1 are connectedin cascade and the terminal line Ln is connected as a two-terminalimpedance in series in the output circuit of the cascade system. Line Lnis open-circuited at its outer terminals. These lines may be portions ofactual transmission lines, for example, coaxial conductor lines, but tosave space it is generally advantageous to construct them as artificiallines, suitable configurations for which are shown in Figs. 2 and 3.

The lines conform to certain general requirements which may be stated asfollows: Those designated L1 to Lm1 are symmetrical, that is, theyexhibit the same characteristics at each of their two pairs ofterminals. They should be substantially dissipationless and should allhave the same propagation constant. Their open-circuit impedances aresimilar in that they exhibit resonances and anti-resonances at the samefrequencies, but the magnitudes of the impedances increasesystematically from left to right in the drawing as hereinafterexplained. The lines should also contain no conductive shuntbranches sothat in the open-circuit condition they are able to store electricalenergy in the form of an electric charge. Line Ln should be similar tothe others With respect to delay time and may also be of similarconfiguration, but since it is used only as a two-terminal impedance,its desired impedance characteristic may be realized in various otherwell-known configurations.

For the generation of impulses of rectangular wave form, it is desirablethat the artificial line sections have characteristics approximatingthose of actual dissipationless transmission lines. Their characteristicimpedances should therefore be substantially pure resistances over awide frequency range and their phase shift characteristics should besubstantially linear in the same range. To produce impulses of a givendesired duration the lines should have a delay time equal to half thelength of the impulse.

Fig. 2 shows a low-pass filter configuration that may be used for thelines, the series inductances and the shunt capacities beingproportioned to place the cut-oil frequency well above the frequencyrange required to produce the d sired pulse Wave form. An alternativeconstruction is shown in Fig. 3 in which the series inductances areprovided by a single long solenoid and in which the shunt condensers areconnected at equal intervals along the inductive winding. The mutualinductance between the successive sections in this construction tends toincrease the linearity of the phase shift characteristic.

Fig. 4 shows a configuration that may be used for the terminatingsection Ln when the other lines are constructed according to Fi 2 or 3.The design of this network may be related to that of the others by meansof the theorem described by R. M. Foster in an article entitled Areactance theorem, published in the Bell System Technical Journal, April1924.

The range of frequencies over which the characteristics of theartificial lines should be uniform depends upon the pulse length andshould be wide enough to include several harmonics of the fundamentalfrequency correspondin to a periodic time of two pulse lengths. Thus, inthe case of a system intended for the generation of pulses of onemicrosecond duration, it is desirable that the artificial lines shouldhave linear characteristics in the range of frequencies extended fromzero to about four or five million cycles per second.

In the operation of the system, when switch I2 is opened, direct currentpasses along the lines to the terminal network Ln and builds up a chargein the capacities included in this network. During the charging part ofthe cycle the circuit is completed through diode 13 which isconductively poled with respect to the charging source voltage. Thevarious transient currents that may arise during this interval are notof present concern since sufiicient time may be allowed to permit thesystem to become fully charged to a steady voltage equal to that of thesource It. Resistance H in series with the direct current source limitsthe transients and serves to dissipate them quickly during. the chargingperiod. The high voltage impulses are produced upon the closing ofswitch 2 by the discharging of the lines through the switch. contactsand through the oscillator diode Id. In so far as the effects producedare concerned, the closing of switch it may be regarded as equivalent tothe sudden application of a steady voltage to the left-hand terminals ofI: having the same magnitude as the source voltage but of oppositepolarity. The transient phenomena during the discharge correspond to thetransmission. of this suddenl impressed reverse voltage, or negativestep-voltage, through the system.

The wave front of the discharge, or of the hypothetical step-voltage,travels through the cascaded lines towards the output circuit and as itsee so, is increased in voltage at each of the junction points becauseof the wave reflection effects arising there. Upon the arrival of thegenerating pulse at the output circuit, current begins to flow in thespace path of the tube l4 and at the same time enters the line 1.71. Thecurrent flow continues at a steady value until the wave entering Lnreturns to the input terminals of that line after being reflected at theopen circuit output terminals. When that takes place the current in theoutput circuit is neutralized by the reflected pulse and the current inthe circuit drops to zero. The duration of the pulse is measured by thetime taken for the generating pulse to travel through the line Lu andback again is thus equal to twice the delay time of that line,

The complete cessation of the current at the end of the assigned pulselength requ res that certain relationships of the parameters of theseveral line sections obtain. This has been men- *ioned before and willnow be explained.

It will be evident that the wave front which travels directly throughthe lines upon the closure of switch l2 will be followed by a successionof wave fronts or step-voltages produced by the reflections at thdifferent junction points in the system. For example, at the junctionpoint of L1 and Le a reflected wave will be transmitted back to theswitch l2 and then after a second reflection will pass through thesystem and will arrive at the output circuit delayed by twice the delaytime of line L1. Similar efiects will be produced at each of the linejunctions and these would ordinarily tend to produce an indefinitelylong train of pulses in the output circuit. It is necessary that all ofthese later pulses should, in effect. neutralize each other so that theoutput current remains zero alter the first pulse has passed. I havefound that this result can be achieved by grading the line impulses andby making the lines all of the same delay time or the same electricallength.

The line impcdances for the configuration shown have the followingvalues: If the resistance of the load, that is, the space path of tube54, be denoted by R, then the terminal line section Ln should haveresistive characteristic impedance of value and the other lines shouldhave impedances of the values given by A demonstration of theserelationships in terms successive wave reflections or transients is 1e,but the relationships may he arrived a consideration of the steady-statechares of the system. The development of ionships on this basis makesuse of cery state theorems on equivalent circuits and the application ofthese theorems to the impulse systems of the invention is justified bythe well-lnown fact that circuits having the same steady-statecharacteristics at all frequencies also exhibit the same transientbehaviors.

The equivalent circuit theorem upon which the demonstration is based isillustrated by Figs. 5 and 6. In Fig. 5 the circuit comprises an idealtransformer I? having a transformation ratio of 1 to 1+7c, a seriesimpedance i8 unrestricted in character and magnitude 70211 and asymmetrical four-terminal network It having an opencircuit impedance Znand an image transfer constant 0. This network is equivalent in itsbehavior to that shown in Fig. 6 which comprises a four-terminal network20 and a series impedance 2|, the parameters of which are related tothose of elements I8 and I9 of Fig. 5, as indicated in the drawing. InFig. 6 the transposition of the series impedance to the other side ofthe four-terminal network and the change in the values of the networkparameters has resulted in the elimination of the transformer. Theequivalence of the two circuits may be demonstrated by developing andcomparing the formulae for the open-circuit and short-cir-- cuitimpedances at each of the two pairs of terminals oi the two systems.

The application of the theorem illustrated by Figs. 5 and 6 to thedevelopment of the systems of the invention proceeds as follows: Thecircuit shown in Fig. '7 is a simple impulse generator of known type inwhich a single rectangular impulse is produced upon the closing of thebattery switch. The line section 22 is a portion of an idealdissipationless line having a delay time equal to one-half the desiredpulse length and having a characteristic impedance equal to theresistance of the load R. The latter requirement is necessary for thegeneration of a single pulse, that is, for the suppression of additionaltransients following the first impulse. In this circuit the pulsevoltage developed at the load terminals is equal to one-half the voltageof the direct current source. Starting with the circuit of Fig. '7, themodification shown in Fig. 8 is arrived at by the insertion of an idealtransformer 24 between the source and the line 22 and by the inclusionin front of the load of a section 223 having the same electrical lengthas line In this figure and in subsequent figures the designation Z isused to indicate the characteristic impedance of the lines. If thecharacteristic impedance of the added line section be such as to matchthe load impedance, the only effect produced by its inclusion in thecircuit will he to delay the appear ance of the pulse at the loadterminals by an amount equal to the delay of the line. The inclusion ofthe ideal transformer gives rise to the desired voltage step-up. Thecircuit arrangement in Fig. 8 now corresponds to that shown in for theparticular case where it is equal to unity. Accordingly, thetransformation shown in Fig. 6 may be used and the circuit of Fig. 9 isarrived at directly. This circuit which produces a voltagemultiplication of two comprises a terminal. line section 25 having acharacteristic impedance equal to half the load impedance and afour-terminal section 25 also having the same characteristic impedance.

By repeating the procedure outlined above as many times as desired,additional line sections of the proper impedances may be inserted andcorresponding increases in the voltage magnification may be obtained. Itis necessary, however, that all of the lines have the same electricallength or delay time in order that the shape and the singular characterof the pulse be maintained. The successive steps in the development ofthe circuit of a voltage tripling system are shown in Figs. 10, 11 and12, respectively. The first step indicated by Fig. 10 consists in theintroduction of an ideal transformer 27 adjacent the voltage sourcehaving a ratio of 1 to 3/2 and the inclusion of a line section 2%adjacent the load and having the same impedance as the load. The circuitin Fig. 11 is derived from that in Fig. 10 by a simple shift of thelocation of the transformer and an appropriate change in the impedanceof the first line section. Fig. 12 then follows from Fig. 11 by theapplication of the equivalent circuit theorem of Figs. 5 and 6. Thefinal impedances of the various line sections are as indicated in thedrawing.

Fig. 13 shows the final result obtained by further application of theprinciple to produce a voltage quadrupling system. Here, the lines havethe impedances from left to right, R, R, R and R. The derivation of thiscircuit from that of Fig. 12 involves the introduction of an idealtransformer having a step-up ratio of 1 to 4/3.

The expressions for the impedance values given in equation 1 may bearrived at by inductive reasoning from a comparison of systems havingprogressively increasing numbers of line sections. The values given forsystems having two, three, and four line sections, respectively, agreewith the values determined from the equation and the values for systemsof greater complexity are readily obtained. The voltage multiplicationin any given system of the invention is equal to the number of linesections used as may be seen from a consideration of the successivevoltage transformations introduced in the course of the development fromthe prototype circuit of Fig. 7. For example, the successivetransformations introduced into the development of Fig. 13 are 2, 3/2,and 4/3, the continued product of which is 4.

The theory of the invention has been developed in the foregoing on theassumption that the line sections are ideal dissipationless lines. Whenartificial lines or networks comprising lumped impedance elements areused, the performance of the system may differ somewhat by that pr0-duced by the use of ideal lines, but if the artificial lines are soconstructed that they have linear phase shift characteristics over asurficiently wide range, the difference in performance will not beserious and will appear primarily as a slight rounding of the corners ofthe rectangular pulse. As already stated, the range in which the networkcharacteristics should be linear should include several harmonics of thefundamental frequency corresponding to a periodic time of twice thepulse length.

Fig. 14 shows an impulse system arranged to generate repeated impulsesand to provide a voltage multiplication of three. The circuit issupplied with energy from. a direct current source 29 which may, forexample, be a rectifier taking alternating current from a power line orother primary source. The discharging switch 12 in Fig, 1 is replaced inthis system by a rotary spark gap 30 driven by a motor, not shown. Thenumber of points on the rotary portion of the gap and. the speed of thedriving motor may be chosen to provide various impulse rates up to about2000 per second. The three artificial lines 3|, 32 and 33 by which thevoltage tripling action is obtained are of the type shown in Fig. 3.Resistance H, charging diode I3, oscillator tube l and output circuitsI5 and I6, correspond to the like numbered elements of Fig. 1. Cathodeheating circuits are indicated for the vacuum tubes,

7 the cathode of tube i4, since it is at a high potential above ground,being supplied with heating current through an insulating transformer 34in the primary leads of which high frequency chokes 35 are inserted.

The source 28 may be capable of supplying a direct current voltage ofabout 20,000 volts in which case an impulse voltage of 30,000 voltswould be developed at the oscillator terminals. This compares with thevoltage of 10,000 that would be produced in the prototype circuit ofFig. '7. The spark gap 30 breaks down and provides a discharge path ofvery low resistance each time one of the rotating points comes intojuxtaposition with the fixed point. Since the pulse duration includingthe delay time introduced by the multiplying network is generally veryshort in comparison with the interval between successive impulses, thesystem can be fully discharged during the passage of the spark and canbe charged again to a steady voltage in the subsequent open-circuitinterval before the next spark.

What is claimed is:

1. In an impulse generating system comprising a pulse shaping networkcapable of storing electrical energy and proportioned to provide animpulse of uniform amplitude and preassigned duration, a load device,and means for discharging the energy stored in said network through saiddevice, means for multiplying the voltage of the generated impulsecomprising a plurality of wave transmission networks interposed intandem between said discharging means and said storage network, saidwave transmission networks having characteristic impedances which varyprogressively from one to the next, the magnitudes of the impedances andthe delay characteristics of the networks being so proportioned withrespect to the impedance of the wave shaping network and the preassignedpulse duration as to substantially suppress transients following theinitial discharge of said shaping network.

2. A system in accordance with claim 1 in which the wave transmittingnetworks are constituted by artificial lines having substantially linearphase shift characteristics and equal delay times, and in which thepulse shaping network has an impedance substantially equal to theopen-circuit impedance of a uniform line having the same delay time.

3. In an impulse generating system for impulses of substantiallyrectangular wave form and preassigned duration comprising a pulseshaping network capable of storing energy in the form of an electriccharge, a load device and means for discharging energy stored in saidnetwork through said load device, means for multiplying the voltage ofthe generated impulse comprising a plurality of wave transmissionnetworks interposed in tandem between said discharging means and saidshaping network, said wave transmission networks having characteristicimpedances the values of which increase progressively from one to thenext in the direction from said discharging means, the impedances andthe delay characteristics of the transmission networks beingproportioned with respect to the impedance of the load device andpreassigned impulse duration to substantially suppress transientsfollowing the discharge of said shaping network.

4. A system in accordance with claim 3 in which the pulse shaping andthe wave transmission networks are constituted by artificial lineshaving delay times substantially equal to onehalf the preassignedduration of the impulse.

5. A system in accordance with claim 3 in which the wave transmissionnetworks are constituted by artificial lines having linear phase shiftcharacteristics and delay times substantially equal to one-half thepreassigned duration of the impulse and having characteristic impedancessubstanitally of the magnitudes defined by and in which the pulseshaping line has a characteristic impedance equal to R+n wherein Zrdenotes the impedance of the rth network of the tandem chain countingfrom the discharging means, R is the impedance of the load device and nis the total number of networks including the pulse shaping network.

SIDNEY DARLINGTON.

